In the power transmission apparatus used for the automotive vehicle, such as for example an automatic transmission apparatus comprises a plurality of planetary gear sets constituting a transmission mechanism and each having a plurality of elements such as a ring gear, a planetary carrier, a sun gear, and a friction engagement apparatus of wet multi-plates type serving as a clutch or a brake. The friction engagement apparatus is operated to have the above elements selectively engaged with and disengaged from each other and a stationary case, thereby changing a power transmission path with a plurality of change stages.
In general, the automatic transmission apparatus is shown in FIG. 5 and comprises a transmission mechanism 100 having a plurality of rotation elements 101 to 103 disposed in parallel relationship with each other. In order to make the transmission mechanism 100 small in size, the distances between the rotation shafts of the rotation elements 101 to 103 are required to be reduced to a level as small as possible.
To meet the above requirement, there has been proposed one of the conventional friction engagement apparatus disclosed in the Patent Document 1 and comprising a transmission case 104, a counter drive gear rotatably supported on the transmission case 104 for outputting a power to the other elements from the transmission mechanism, and a piston forming part of a friction brake apparatus and formed roughly in a cylindrical shape surrounding the most part of the counter drive gear. The piston is formed with a cut-off recession cut off to extend in its axial direction for allowing the counter drive gear to partly be received in the cut-off recession to ensure that the counter drive gear is held in mesh with a counter driven gear for outputting the power. The counter driven gear is also partly surrounded by the piston and partly received in the cut-off recession. The piston of the conventional automatic transmission apparatus is produced by a press working to have a bottom and a thin thickness so that the piston can have a relatively high mechanical strength. The piston is operated with a return spring in the form of a plate spring to ensure that the axial length of the friction engagement apparatus is shortened.
Another conventional friction engagement apparatus is disclosed in the Patent Document 2 and comprises a snap ring formed in a C-shape and having an opening along the periphery thereof in the neighborhood of friction plates. The snap ring is arranged with respect to a pair of power transmission gears with the opening held in registry with an opening formed in a case to allow the power transmission gears to be held in mesh with each other through the opening of the snap ring and the opening of the case so that the distance between the both axes of the power transmission gears can be as small as possible.
Still another conventional friction engagement apparatus is disclosed in the Patent Document 3 to comprise a return spring for resiliently urging a piston to its releasing state to have a brake opened. The apparatus has an annular retainer partly cut-off to have a cut-off recession through which the pressure imparting portion of the piston is projected. The other conventional friction engagement apparatus is disclosed in the Patent Document 4 and comprises a plurality of annular friction plates each functioning as an annular brake plate.
The brake plate has a plurality of external teeth formed on the peripheral portion thereof except for a peripheral portion cut-off with no tooth portion, i.e., a groove receiving therein a set of return springs. Each of the return spring is composed of a compression spring to have the friction engagement apparatus prevented from becoming large in size.
Patent Document 1: Patent Publication No. 2006-144879
Patent Document 2: Patent Publication No. 2000-81118
Patent Document 3: Patent Publication No. 2007-170442
Patent Document 4: Patent Publication No. 2007-170440
However, the urging means in each of the above mentioned conventional friction engagement apparatuses thus constructed is designed to urge the piston toward the piston releasing state, the urging means including a plurality of compression coil springs circumferentially equally spaced apart from one another on the piston, or including a return spring made of a plate spring and having a spring stiffness and a plate bending stiffness both of which are equal at its circumferential positions of the piston.
In the conventional automatic transmission apparatuses, the piston is required to be partly cut-off to have a cut-off recession extending in the axial direction for the purpose of avoiding the counter drive and driven gears and other elements from interfering with one another. However, the piston assembled in the automatic transmission apparatus is imparted a circumferentially equal pressure by the compression coil springs spaced apart from one another on the piston and by the return spring having a spring stiffness and a plate bending stiffness both of which are equal at its circumferential positions of the piston. The circumferentially equal operation fluid pressure exerted on the piston leads to the fact that the cylindrical portion of the piston in the vicinity of the cut-off recession is susceptible to the concentrated stress, thereby making the piston apt to be deformed if the piston is made small in size.
The present invention has been made to solve these problems encountered by the conventional friction engagement apparatus. It is, therefore, an object of the present invention to provide a friction engagement apparatus comprising a piston restrained from unnecessarily deformed even with the cut-off recession formed in the piston to ensure that the piston becomes small in size, thereby making it possible to provide a friction engagement apparatus suitable for the power transmission apparatus also small in size.